The objective of this research is to examine under which conditions sub-surface entrainment (winnowing) of particles occurs and to estimate which parameters are governing the winnowing process. The investigation is executed with laboratory experiments.

Laboratory tests has been performed with specific material types, namely M32 sand (D50 = 370 ìm) and Carara gravel (D90 = 23 mm). Model tests performed by Detert et al (2010), show that the lift intensity decreases exponentially vertically through the porous bed. According to Diplas and Parker (1992), the surface layer thickness can be estimated as 2-3 times of the surface material size D90. Applying this assumption, two values of the surface layer thickness are investigated, a surface layer thickness of 6.4 cm (2.8 D90) and of 4.6 cm (2.0 D90). The study of Cui (2007) shows that an increase of entrained fines can be achieved by generating a higher surface layer mobility. For this experiment, the layer thickness and surface layer mobility are varied to investigate their influence on the winnowing process.